Mass spectrometric study of the vaporization behaviour of α-Na2NpO4: Thermodynamic investigation of the enthalpy of formation

The work herein presents the first high temperature Knudsen effusion mass spectrometric measurements carried out on the sodium neptunate α-Na2NpO4. The material’s vaporization behaviour was investigated under vacuum conditions, in a tungsten metal cell up to T = 2700 K, and in an alumina cell up to T = 1900 K. Gaseous and condensed phases are in equilibrium under Knudsen conditions. The equilibrium decomposition reaction of α-Na2NpO4 to neptunium oxide, sodium oxide, and oxygen was established in the temperature range 1030 K to 1206 K. The enthalpy of formation of α-Na2NpO4 at 298.15 K was furthermore estimated at (−1761.0 ± 7.7) kJ·mol−1 using a second law analysis in the interval 1030 K to 1115 K, in good agreement with the value reported by Goudiakas et al. [25] (−1763.9 ± 7) kJ·mol−1, who used solution calorimetry. In the interval 1115 K to 1206 K, where the vaporization of sodium oxide formed by decomposition of α-Na2NpO4 is considered, the analysis yielded at 298.15 K (−1761.9 ± 20.4) kJ·mol−1 and (−1758.8 ± 22.3) kJ·mol−1 taking into account two different sodium oxide sublimation mechanisms suggested by Hildenbrand and Lau [56] and Steinberg and Schofield [37] respectively. Finally, the detection of the masses 253 and 292 in the temperature range 1350 to 1620 K suggests a probable sublimation of the high temperature tetragonal phase of the Na2NpO4 compound in parallel with the decomposition. The method developed in the present paper holds promise for the thermodynamic investigation of other sodium neptunates and plutonates phases.

► α-Na2NpO4 decomposes to NpO2, Na2O and O2 between 1030 and 1206 K under vacuum.
► The enthalpy of formation of α-Na2NpO4 at 298.15 K is (−1761.0 ± 7.7) kJ.mol−1.
► The sublimation of Na2O probably involves all species Na, O2, NaO, Na2O, NaO2.
► We found evidence that Na2NpO4 sublimes between 1340 K and 1620 K.